1. Field of the Invention
The present invention relates to a process for the recovery of helium from gas streams containing high concentrations of hydrogen and low concentrations of helium. More particularly, it relates to the recovery of helium and hydrogen from feed gas streams containing high concentrations of hydrogen and low concentrations of helium by first removing a hydrogen rich gas stream to form a hydrogen depleted feed stream and then recovering a helium rich gas stream from the hydrogen depleted feed stream. The hydrogen rich gas stream can be recycled or recovered and the helium rich stream can be further refined to produce high purity helium gas.
2. Background of the Invention
Helium is a rare gas. It has unique physical properties making it valuable in a wide range of commercial applications. For example, helium is inert, it's boiling and melting points are the lowest among the elements, it is the second lightest element, and it only Exists as a gas in all but the most cryogenic conditions. Commercial applications range from balloons to lasers and include heat transfer and cooling applications; shielding applications; gas tracing applications; inert environments for growing crystals; and numerous other applications. However, due to its limited availability and increasing commercial demand, its costs have increased significantly over that the last few years.
While helium is the second most abundant element in the universe, it is extremely rare on Earth. In fact, it is a finite resource and is primarily found trapped with natural gas in concentrations up to about seven percent by volume although most often, it is found in concentrations of less than 0.5% by volume. Almost all of the commercial helium requirements are supplied by the extraction from helium-containing natural gas by low temperature fractional distillation processes. The resulting helium rich gases are further purified or refined using additional cryogenic distillation steps or by pressure swing adsorption (PSA) processes which selectively remove other gases. These final refining steps result in commercial grades of helium in excess of 99.9%.
The high cost and limited availability of helium gas has resulted in investigations to identify other sources of helium. The source most relevant to the present invention is hydrogen containing gas streams produced from or existing in various current commercial processes. Process gas streams such as those from the production of hydrogen, or which recycle hydrogen, can contain helium in concentrations which can be economically extracted and recovered. For example, processes for the production of synthesis gas (CO/H2), which is typically further used in hydrogen production, ammonia production or used in certain Fischer-Tropsch reactions, typically begin with natural gas feed streams. The various processing and recycling steps do not normally separate the inert helium already present from the natural gas feed stream and it often accumulates in the processing steps. The resulting recycle and purge gas can contain helium in concentrations of up to 2% or more by volume.
Of particular interest for the present inventive process is the use of a process gas stream that is vented or recycled during the production of ammonia. Ammonia is produced commercially in vast quantities with worldwide production estimated to be nearly 150 Metric Tonnes per year. The most common method of production is the Haber process in which nitrogen is combined directly with hydrogen at high temperatures and pressures in the presence of a catalyst. The hydrogen used in the process is typically produced from natural gas, as discussed above, and the reacted gas stream containing predominantly hydrogen, methane and nitrogen and smaller amounts of argon, helium and traces of other gases is typically recycled to the reaction chamber. The concentration of the various species, particularly methane and argon, will depend on the location in the loop from where the gas stream for helium recovery is tapped. Helium, as well as other inert gases, accumulates in the recycle gas stream and this accumulated concentration of helium makes the recycle gas stream a useful feed stream for helium recovery. Further, to prevent build-up of inert gases in the recycle loop in a typical ammonia production process, a purge is established which is rich in hydrogen and can have helium in concentrations worth recovering. This gas stream containing predominantly hydrogen and nitrogen and containing up to 10% helium is a particularly useful feed for the present process.
Thus, the present invention recovers helium from gas streams containing high concentrations of hydrogen and low concentrations of helium, separates and recovers a high purity hydrogen stream for further processing or for another use, and subsequently recovers a helium rich gas stream that can be further processed using conventional helium refining techniques. Unlike conventional processes, this process advantageously recovers two economically valuable gas streams; one being a helium enriched gas stream and the other being a high purity hydrogen gas stream.
Processes using multi-step cryogenic distillation or hybrid PSA systems for removing or extracting helium from natural gas or other gases containing low concentrations of helium are known. These processes are typically directed to natural gas feed streams and require significant capital investments and complex and expensive cryogenic distillation systems to generate the enriched helium gas streams which can be further processed or integrated into the starting extraction process. Similarly, related processes for the recovery of hydrogen from refinery off gases have been taught.
In U.S. Pat. No. 3,807,185, helium containing vent gas from the synthesis of ammonia is subjected to several partial condensation steps and then cooled to remove the higher-boiling components, namely, methane, nitrogen, and argon in a phase separator. A low temperature stripping column is then employed to recover liquid hydrogen and a gaseous helium/hydrogen mixture. Hydrogen and argon can also be recovered. This process requires multiple cooling and heating steps and, the hydrogen feed stream must be condensed at extremely low temperatures requiring high capital equipment costs and high power requirements.
U.S. Pat. No. 5,632,803 discloses a process in which a membrane separation is followed by two stages of PSA processing used in series to recover helium from source streams containing 0.5% to 5% by volume helium in concentrations of greater than 98% by volume. This process is directed to recovering helium from a gas stream containing primarily hydrocarbons and some nitrogen and is not economically desirable for separating helium from a feed stream containing mostly hydrogen because the PSA adsorbents typically have low capacities for both hydrogen and helium and have poor selectivity for hydrogen over helium.
U.S. Pat. No. 6,179,900 discloses a process wherein the feed gas contains a desired component in low concentrations, preferably hydrogen from FCC gases. The feed gas is first passed through a membrane separation stage to produce a permeate gas enriched in the desired component. The high pressure retentate gas is used to compress the low pressure permeate gas which is then fed to a PSA to purify the desired component.
U.S. Pat. No. 4,360,505 discloses an adiabatic process for recovering hydrogen from mixed gas streams using hydridable materials as the absorbing medium. The process comprises utilizing a composite of a thermal ballast in admixture with the hydride material to absorb the heat of reaction and to aid in desorption.
U.S. Pat. No. 5,771,714 discloses a process for the production of helium from a feed gas comprising helium, nitrogen, and hydrocarbons in a system comprising a high pressure and a low pressure cryogenic rectification column.
None of these processes contemplate using a feed gas stream having a high concentration of hydrogen, such as above 50% by volume, removing a higher purity hydrogen stream and then separating helium from the hydrogen depleted gas stream to obtain a helium rich product gas stream in the absence of either a cryogenic distillation or a PSA step. The present process provides for a unique, ambient temperature process for recovering both a helium rich product stream and a high purity hydrogen product stream in an integrated process and in a cost effective manner. Prior processes that recover only one of these gases and do not effectively separate hydrogen from helium do not take advantage of the low cost integration and incur significant economic penalty.
Thus one objective of the present process is to recover a helium enriched gas stream from a feed gas stream containing a hydrogen concentration in excess of 50% by volume and helium concentration less than 10% by volume which is further processed to produce a helium rich gas stream at concentrations of helium of greater than 25% by volume while at the same time recovering a higher purity hydrogen gas stream at concentrations of hydrogen of greater than 90% by volume.